Experiences on the Shortwaves

08/19/2015 08:30

Wide Band MOPA

It is not necessary to have a digital, modern, transceiver or some surplus radio set to transmit on the Shortwaves on any frequency. Being fascinated by valve radios, I have found an alternative, rather simple, that uses electron tubes and elementary circuits: the MOPA, which means, essentially, a two stages transmitter, made be a Master Oscillator and a Power Amplifier.

For many years in the past the MOPA represented the starting point for any newly licenced Radio Amateur, to operate on the Shortwaves. QST and the ARRL Handbook offer some very nice examples of such transmitters. All of them need Crystals to generate the driving signal. Nevertheless, it is little practical to go 'rock-bound' if one wants to surf the aether, being free to change frequency to avoid making QRM or for exploiting the optimal band.

My humble solution, then, uses a simple analogue VFO which provides the necessary signal, through an added couple of almost-aperiodic amplifiers. The whole chain being made, rigorously, by valves!

A roperly buffered VFO and with properly stabilized plate and filament voltages, allows to achieve a very stable and clean tone on Radiotelegraphy. A MOPA works with frequency multiplication to produce higher frequencies. Any drift in the VFO signal is, then, multiplied, and so worsened. It can be easily understood how it is important to have a stable VFO, then. You might use a keyed DDS (Digital Direct Synthesizer), but ... where's the fun and the simplicity, then? Lost ...

Despite the many statements found in the literature, if a VFO is properly built, electrically and mechanically, then it is most convenient to key it on Radiotelegraphy: keyed VFOs allow a true side tone, so your own emission can be monitored on you receiver.An oscillating triode, with a keyed negative bias on the control grid, and fed by relatively low plate voltages, is almost 'click-free' and 'chirp-free'. My VFO triode, for example, is fed at 35-40 VDC keydown.

Then, two buffer amplifiers are needed, as stated above. The first, receives the signal directly from the VFO, and is a 'cathode follower': its outuput is taken from the cathode, at low impedance. Only a cathode follower can guarantee the best, first, isolation of the VFO to the stages downstream.

Then, to provide the 'Master Oscillator' tube with an adequately high drive level, so that, in turn, when it works as frequency multiplier it can deliver adequate power to the Power Amplifier stage, you need a second buffer. This one must be, then, a 'common cathode' stage, which only is capable of maximum voltage amplification.

Both buffers are almost wideband. If no amplitude compensation network is used to raise the buffers chain gain for higher input frequencies, generally at the second buffer output the signal amplitude could be well below the same VFO output level, making the frequency multiplication in the driver stage not efficient.

The solution which came into my mind was to apply an 'L-R-C' network, properly configured, in correpondence of the second buffer's plate. LT SPICE IV circuit simulator, has been an essential tool to evaluate what could be the overall performance. Here below, the simplified, small signals, schematic of the connection between the 2nd buffer and the driver tube. The '250 uH' coil is the plate chocke towards the plate supply.

And here it is the frequency response (solid line) of this transmitter circuit portion.

The peak response at 2.5 MHz depends on the interaction of the plate chocke with all the circuit capacitances, and is so high that, actually, causes instability and tendency to self oscillation. The 250 uH chocke, then, must be damped with a parallel resistor, as it is shown later in the modified circuit.

The reality was just like that: with a 9 MHz signal, 1.0 Vpeak developed by my VFO, it was not possible to properly driver the PA stage, which delivered only a few Watts on higher Shortwaves.

Later, I learned from schematics of professional vintage radio equipment, that both the input and output valve capacitances, could be conveniently used in resonating networks, for amplitude-frequency compensation purposes. Then, by a wise use of LT SPICE, I ended to the following modified circuit, which raised the frequency reponse in the 10-11 MHz range, ... a critical band for the reasons that are explained below.

The concept here is to place in series (!) to the inter-stage coupling capacitor (150 pF in my transmitter) a conveniently chosen inductor (56 uH, from my simulations), to resonate with the 8pF and 10 pF input and output tubes capacitances.

By experience I've noticed that the best compromise between driving level requirements and worsening of VFO defects, is to have a driver stage that multiplies the frequency at maximum by a factor 3. So, to get on CB band and the 10 mt band, the VFO is required to generate a 9.5 MHz signal with decent features. Which is rahter easy, by the way. A cascade of two R-C coupled valve amplifiers (cathode follower class A amplifier driving a common cathode class A amplifier), soon attenuate the input signal in the range of 8 MHz and above ... So I needed some wide-band amplitude boost network
JUST from 8 to 10 MHz, and here's what I've come up with.

The results shown by very approximate simulations are confirmed in the reality and about 10 dB of drive level increased have been achieved.

My modified general coverage MOPA, is driven by a triode VFO (half of a 6BQ7 tube) that oscillates in 6 bands, from 1.6 to 3.6 MHz and from 6.8 to 9.5 MHz. By doing X1, X2 and, at worst, X3 in the driver stage, any frequency from 1.6 to about 30 MHz, with very nice and stable CW tone quality, can now easily achieved.

The first buffer is a cathode follower triode (the remaining 6BQ7 half). The second buffer and the driver are made by EL83/EL803/EL803S tubes, cousins of the most known 12BY7 or 6CL6 (refrain to use a 5763: ... not enough amplification for such specifications!). Then, the PA is made by a pair 6BG6G sweep tubes, twin sisters to the most known 807, apart from the socket.

The overall performance, with un-optimized PA impedance network, is such that on lower HF up to 10 MHz, 50 Wrms are delivered to the antenna, descending then downto 25 W on 24/27/28 MHz bands. A very respectable performance for a VFO-driven-and-VFO-keyed MOPA.

Isn't it ?

... no need here of sofisticated or relatively modern radios ( :-P ). Just a few (old) components and a wise use of the modern simulation technology, which, anyway, does help to still squeeze out the maximum from our poor valves.

And yet ... I can surf on all the Shortwaves without any limitations ...